1. Field of the Invention
The present invention relates to a semiconductor condenser microphone chip.
2. Description of the Related Art
A condenser microphone chip is a capacitor composed of a diaphragm and a backplate. Currently, in most reports and patents a double-membrane capacitor structure which is manufactured by forming a diaphragm and a backplate on a silicon wafer by micromachining is adopted. Few efforts are made to the development of single-membrane silicon condenser microphone. A single-membrane silicon condenser microphone is reported in “Fabrication of Silicon Condenser Microphone Using Single Wafer Technology”, Journal of microelectromechanical systems, VOL. 1. No. 3, 1992, p 147-154. In the single-membrane silicon condenser microphone, a capacitor structure is formed by an edge portion of a diaphragm and a silicon substrate with the silicon substrate serving as a backplate and with a large hole at a center of the backplate serving as a sound hole. However, the single-membrane silicon condenser microphone is disadvantageous because an edge of the diaphragm is connected to a peripheral portion. When a sound wave is applied to the diaphragm, a maximum vibration occurs at a center portion of the diaphragm, and a small vibration is generated at the edge portion of the diaphragm. Because the center portion of the diaphragm is directly opposite to the sound hole of the backplate, mechanical sensitivity in the region with maximum amplitude is not used, so that the mechanical sensitivity of the diaphragm contributes less to sensitivity of the microphone.
In order that a diaphragm has good vibration performance, a residual stress in the diaphragm can be reduced. In the Publication titled Sensor and Actuators A. 31, 1992, 90-96, a material with tensile stress and a material with compressive stress are used to make a low-stress composite membranes for a microphone. In U.S. Pat. No. 6,622,368B1 in which silicon nitride/polysilicon/silicon nitride composite membrane structure is disclosed, the low-stress composite membrane is used as a diaphragm of a microphone. In the Publication “Sensor and Actuators A. 31, 1992, 149-152” and U.S. Pat. No. 6,012,335, a monocrystalline silicon diaphragm is made by doping monocrystalline silicon with boron. In the Publication “A High Sensitivity Polysilicon Diaphragm Condenser Microphone”, 1998 MEMS Conference, Heideberg Germany January 25-29, it is reported that a diaphragm is made with low-stress polysilicon. However, requirements for a growing process of a membrane is strict and it is difficult to assure uniformity of the membrane if vibration performance of the diaphragm is improved only by making a material of low residual stress.
In addition, the methods for releasing residual stress in a diaphragm with various structures have been known in the art. In the Publication U.S. Pat. Nos. 5,452,268 and 5,146,435, Chinese Patent Publication No. 1787693A, and a literature (The 11th International Conference on Solid-State Sensors and Actuators, Munich Germany, Jun. 10-14, 2001), mechanical sensitivity of a diaphragm is improved by releasing residual stress in the diaphragm by using a cantilever structure. Since stress in the diaphragm is concentrated at an edge of the diaphragm due to the cantilever structure and the beam structure is often too soft, an adhesion problem is apt to occur. In U.S. Pat. No. 6,535,460 B2, a free diaphragm structure is disclosed. With the free diaphragm structure, a microphone with a diaphragm of residual stress of zero can be obtained, but a process required for preparing the structure is complicated.
A rigid backplate is a premise for a microphone having good frequency characteristic and low noise. Currently, methods for making a rigid backplate comprises: employing a thick gold layer as a backplate in U.S. Pat. No. 6,012,335; employing a composite metal membrane as a backplate, which increases thickness of the backplate while decreasing stress in the backplate, in U.S. Pat. No. 6,677,176 B2; employing a monocrystalline silicon layer in a SOI silicon wafer as a backplate in U.S. Pat. No. 6,140,689; employing electrochemical corrosion to make a low-stress thick monocrystalline silicon backplate in U.S. Pat. No. 6,667,189 B1; and making a particular structure to increase strength of a backplate in U.S. Pat. No. 6,532,460 B2. However, most of the above processes are complicated and are high in manufacturing cost.
After a soft diaphragm and a rigid backplate are obtained, it is also necessary to solve the problem that the diaphragm is attached or adhered to the backplate. Up to now, there have been many methods. An effective method is to make attachment or adhesion preventing protrusions, but it is necessary to increase a number of processing steps and thus cost.
In U.S. Pat. No. 5,870,482, a cantilever beam type diaphragm is described. A cantilever beam is fixed at an end, and constitutes a capacitor at an edge portion of a free end with a backplate. With the above configuration, mechanical sensitivity makes great contribution to microphone sensitivity, but structure of the diaphragm is complicated. In addition, because of the cantilever structure having 3 DOF (Dimension of Freedom), it is difficult to assure pose and reliability of the diaphragm. In U.S. Pat. Application Publication No. 2006/0093170 A1, a single membrane structure in which outer cantilever beams are distributed at equal intervals is disclosed. An edge portion of a diaphragm and the backplate form a capacitor. The cantilever beams improve contribution of mechanical sensitivity to microphone sensitivity, but can not enable the diaphragm to translate. In addition, with the above configuration, it is difficult to assure yield and reliability.